The present invention is directed to switching mode power supplies (SMPS). More particularly, embodiments of the invention provide circuits and methods of transferring system output status through the transformer. Merely by way of example, the invention has been applied to control the output voltage undershoot of a pulse width modulated (PWM) or a pulse frequency modulated (PFM) switching mode power supply for reducing the output voltage undershoot of switching mode power supplies during the load transient of the SMPS. But it would be recognized that the invention has a much broader range of applicability.
Switching mode power supplies (SMPS) have the advantages of smaller size, higher efficiency and larger output power capability, and are widely applied in mobile phone chargers, notebook computer adapters and other fields. In recent years, green power supplies are emphasized, which require higher conversion efficiency and especially lower standby power consumption. This leads to the continuous reduction of switching frequency in light load or no load conditions of an SMPS, because lower switching frequency in light load or no load conditions can reduce switching loss of power transistor, thus increase power conversion efficiency and reduce system standby power. However, reducing switching frequency in an SMPS can also lead to many limitations. Some of these limitations are described in more detail below.
In view of the foregoing, there is a need for an effective technique to reduce the output voltage drops during load transients or dynamic load changes of switching mode power supplies.